The present invention relates to novel copolymers suitable for preparing contact lenses and contact lenses made thereof which do not substantially absorb water and have an excellent oxygen permeability enough to wear them continuously for a long period of time.
Contact lenses put presently on the market are classified into two large groups of water-nonabsorptive contact lenses and water-absorptive contact lenses. The water-nonabsorptive contact lenses are further classified into hard contact lenses made of polymethyl methacrylate and soft contact lenses made of silicone rubber. Also, as the water-absorptive contact lenses, soft contact lenses made of poly-2-hydroxyethyl methacrylate and high water content contact lenses made of polyvinyl pyrrolidone are known.
The water-nonabsorptive hard contact lenses made of polymethyl methacrylate have a history of several tens of years, but are still insufficient for use in a clinical viewpoint. That is to say, although such contact lenses have excellent optical property and durability, they give a strong foreign body sensation to persons in an early stage of the wear due to being poor in hydrophilic property of the lens surface and, therefore, require a long term for accommodation to the lenses. Moreover, since the oxygen permeability is poor, it is impossible to wear them continuously for a long period.
The silicone rubber soft contact lenses are very water-repellent and are greatly different from cornea in thermal properties such as thermal conductivity and thermal diffusivity. Therefore, they give a foreign body sensation, particularly a burning sensation, despite of having a very large oxygen permeability. A stronger patience than the case of polymethyl methacrylate lenses is required for accommodating thereto. Also, silicone rubber is soft and elastic, and precise mechanical treatments such as cutting, grinding and polishing are very difficult. On the other hand, many attempts to make the surface of silicone rubber lenses hydrophilic have been reported, but satisfactory silicone rubber contact lens has never been developed.
The water-absorptive contact lenses made of poly-2-hydroxyethyl methacrylate are good in a sensation of wear, but since the water content of the lens is at most 40% by weight, the permeation of oxygen through water as a medium is insufficient and it is impossible to continuously wear the lenses for a long period of time.
The high water content contact lenses made of polyvinyl pyrrolidone have an excellent oxygen permeability. However, they have the disadvantages that the durability is poor, and also that the water content of the lens changes depending on external environment at the time of wearing the lenses and the lens contour changes with the change of the water content so as to lower the visual correction ability.
Also, there is a serious problem in these water-absorptive contact lenses that they are contaminated by bacteria. Although boiling treatment, chemical sterilization treatment, etc. are considered as a means for preventing the contamination by bacteria, these treatments are troublesome. Moreover, the boiling treatment accelerates the deterioration in quality of lens material and the chemical sterilization treatment incurs danger to eyes.
As stated above, conventional contact lenses have various defects, and a contact lens having no such defects are desired.